1. Field of the Invention
This invention relates to package tying machines, and more specifically to the twine tensioning devices of such machines. This invention is in the nature of an improvement upon the machine disclosed and claimed in U.S. Pat. Nos. 1,606,290, dated Nov. 9, 1916; 2,371,024, dated March 6, 1945; and 2,898,847 dated Aug. 11, 1959.
2. Description of Prior Art
Reference is made to the above identified patents and also to U.S. Pat. No. 2,873,664, dated Mar. 25, 1954, wherein the structure and operation of a spring controlled twine tensioning device, as well as the overall operation of a package typing machine, are disclosed and described. Proper tensioning of the tying medium (twine, tape, or other tie material), of course, is necessary to assure a properly tied knot and that the tying medium is sufficiently taut around the package.
It is sufficient to note that prior art package tying machines utilized a spool of twine from which a free end passed up through a series of guides, on through a twine tensioning device, through the machine's hub and rotatable twine arm, and on through another series of guides and rings until held by a stringholder mechanism disposed in proximity to the knotter mechanism of the tying machine.
With specific reference to U.S. Pat. No. 2,898,847, the typical prior art tensioning device disclosed therein was mounted between the twine supply and the hollow shafted twine arm and utilized a flat plate with an opening through which the twine could pass. The twine was biased against the flat plate by a leaf spring to establish that the proper tensioning of the twine. The leaf spring could be pre-adjusted to a proper spring force level as operating conditions required.
The leaf spring in that type of tensioning device merely yieldably moved out of the way when a twisted, bunched or otherwise thick portion of the material passed thereby. However, such prior art devices and tensioning methods made no provision for overcoming the problems generated by the varying tension values of the tie material which resulted from spring force variations of the tensioner. These spring force variations were caused by the constant moving of the spring in response to variations in tie material thickness.
This tension variation problem is of special concern when the so-called "poly tape" type of tying medium is utilized, such as, for example, polyethlene or poly florin tape. Unlike twist, poly tape has unique characteristics during a tying cycle in that it tends to curl and fold over upon itself from an initially flat condition as it unrolls off the supply roll to a multiply-twisted ribbon. Accordingly, when entering the twine tensioner of the prior art tying machines, poly tape had a constantly changing thickness and volume.
As noted above, any variation in poly tape thickness--from a few thousandths of an inch up to a few hundredths of an inch--resulted in a varying tape tension throughout the tying cycle since the spring controlled twine tensioner was pre-set to operate at a constant spring force level for a constant tape thickness. Because of the variable tension problem, prior art tensioners had to be set so that the maximum tape tension resulting from the thickness variation would not pull the tape from under the tying machine's stringholder button. This ultimately resulted in an average tape tension that was far less than the strength of the tape would otherwise allow. This in turn resulted in undesirable wrapping and knotting conditions for poly tape.
An additional problem with the prior art leaf spring/flat plate type tensioning devices, including the device disclosed in U.S. Pat. No. 2,898,847, is that utilization of twine therein required use of a laterally radiused leaf spring to confine and maintain the twine underneath the longitudinal center line of the spring. This was done to keep the twine from tending to escape sideways or squirt out from under the pressure of the leaf spring at the latter's point of contact with the flat plate. However, utilization of poly tape with the same laterally radiused leaf spring produced ineffective tape tensioning as the tape would simply pass within the lateral radius at the spring's contact point. This required the provision of one type of tensioning device for use with twine and another type of tensioning device for use with poly tape materials.
An additional problem is that, while twine is relatively limp and when tensioned will still easily pass through the three or more required right angle bends of the hub assembly and twine arm, poly tape becomes rather rigid and stiff under tensioning. The result is that excessive drag is placed on the poly tape when passing through the various bends of the twine arm. This problem of excessive drag in poly tapes, when tensioned prior to entry to the twine arm, required a further undesirable reduction in the flat spring tension in prior art tensioner devices so as to not pull the tape out of the stringholder button mechanism.